1. Field of the Invention
The present invention relates generally to an improved plasma etching process for etching patterns in metal films, and more particularly to a process for aiding in the fabrication of MOS and bipolar integrated circuits and Schottky devices such as Schottky transistors.
2. Description of the Prior Art
Examples of patents which relate to etching patterns in metal films or to formation of Schottky barriers by use of a plasma are U.S. Pat. No. 3,795,557, issued Mar. 5, 1974 to A. Jacob, and U.S. Pat. No. 3,879,597, issued Apr. 22, 1975 to Bersin, et al; disclosures of these two patents are incorporated herein by reference.
Metal-semiconductor rectifying devices have been investigated since the late 1800's. Sometime thereafter, around the turn of the century, a point-contact rectifier (metal-semiconductor junction device) may have found practical utility as evidenced by U.S. Pat. No. 775,840, issued in 1904 to J. C. Bose. In the late 1930's W. Schottky continued to study the metal-semiconductor potential barrier, and the suggestion that this potential barrier could arise from arrangement of space charges within the semiconductor, not necessarily requiring presence of an adjacent chemical layer, is attributed to him. This development continued through more recent history, and the metal-semiconductor junction rectifier presently employed in electronic circuit applications is commonly called a Schottky diode or Schottky transistor.
The Schottky barrier contact or interface, as noted, is a rectifying metal-semiconductor junction. Such Schottky barrier contacts utilize the Schottky effect based upon rectification characteristics exhibited by well known metal-semiconductor interfaces. Generally, electrical characteristics of these contacts depend upon what is termed "work function" of the metal, as well as electron affinity in the semiconductor material. The work function is generally defined as minimum energy necessary for an electron to have in order to escape into vacuum from initial energy at the Fermi level, in a metal-vacuum system. In other words, in a metal, the Fermi level is an energy reference level from which an electron is removed to the free state (vacuum electron) by an amount of energy equal to the work function.
Fabrication of these Schottky barriers has been subject to various improvements over a period of time. Presently, plasma etching is employed in the fabrication of these semiconductor devices, as well as others.
In plasma etching of patterns in metal films on a semiconductor substrate (these metal films being used in Schottky devices and for interconnecting various regions within integrated circuits), a variety of metals can be processed. For one example, a metal film layer can be comprised of a base layer of titanium, with another layer of tungsten thereon, and beyond that, a top layer of aluminum.
In the prior art, the desired pattern is etched first in aluminum by using conventional photolithography and chemical etching techniques. In this prior art approach, the photoresist layer is then removed prior to subjecting the wafers to plasma etching, (the plasma being generated by equipment containing an etch tunnel, as for example, shown in the references cited above). The plasma ambient used to etch titanium, tungsten, and silicon films consists of primarily carbon-tetra-fluoride (CF.sub.4). The etch tunnel in the plasma etching equipment yields atomically neutral species of fluorine, which react with all of the above films, except the aluminum outer layer. Volatile compounds with fluorine are produced, and hence a pattern is etched in the films leaving behind the metal under the aluminum.
In the above prior art process, problems are encountered because of the aluminum/plasma interaction when narrow spacings (less than or equal to 2 mils) are to be etched. These problems are often encountered in various integrated circuits, high frequency transistors, and printed circuit board applications, because these devices may have quite narrow spacings. Etching of films then becomes incomplete in the narrow spacings. These incomplete etched films thus provide "bridging" interconnections between different metallic parts of the device, thereby creating short circuits and otherwise reducing or inhibiting performance of the device.
One of the prior art solutions was to attempt to complete the etchings in these narrow regions by over-etching until the entire field becomes clear. However this prior art solution gives rise to additional problems as for example (1) undercutting of the pattern in the regions other than those with narrow spacings, and (2) creating shorts and leakages by overetching certain regions such as n+ cross under, and phosphorous doped glass in the emitter, thereby making them thinner than normal.
These are severe problems of the prior art process. Applicants have discovered and invented an improved process to fabricate a variety of integrated circuits and Schottky transistor devices, which improved process eliminates above-described severe shortcomings of the prior art process.